1. Field of the Invention
The present invention relates to a wide nip calender arrangement for glazing a paper or cardboard web that includes at least one wide nip formed between a back pressure surface and a circulating jacket loaded by a support shoe arrangement in the direction of the back pressure surface, and a calibration nip. The invention further relates to a process for glazing a paper or cardboard web in which the paper or cardboard web is guided through a wide nip formed between a back pressure surface and a circulating jacket loaded by a support shoe arrangement in the direction of the back pressure surface, and the paper or cardboard web is calibrated.
2. Discussion of Background Information
In the manufacture of paper or cardboard webs, the situation frequently occurs that the paper or cardboard web exhibits a relatively nonuniform thickness across its width. Homogenizing this thickness in the paper machine is usually associated with a relatively large expense, so that it is preferred to homogenize the thickness in a calibration nip behind the paper machine. If the thickness were left nonuniform crosswise to the travel direction of the paper or cardboard web, problems would arise in the further handling of the paper or cardboard web. This can be observed in particular during reeling of the paper or cardboard web. If the paper web is thicker at one end (seen from the width direction) than at the other end, such clear differences in diameter occur with several hundred or even several thousand windings of a paper or cardboard roll, that this paper or cardboard roll can no longer be rolled up well and further processed.
For surface refining with a paper or cardboard web, it is preferred to use a wide nip press in which the paper or cardboard web can be acted on over a predetermined distance by low to increased pressure and, if necessary, also with an increased temperature with the object of keeping the loss in volume as low as possible during condensing. The wide nip press forms a wide nip in which a circulating jacket is opposite a back pressure surface. The jacket is pressed by a support shoe arrangement against the back pressure surface, so that a correspondingly greater or lesser pressure is exerted on the paper or cardboard web between the jacket and the back pressure surface. The wide nip has the advantage that the paper or cardboard web is acted on with pressure over a longer period. However, the compressive stresses in the wide nip are comparatively small, so that a volume-conserving glazing is possible.
In many cases it can be observed that the paper or cardboard web undergoes a powerful drying in the wide nip. Therefore, it has been suggested to arrange a moistening device before the wide nip, so as to moisten the paper or cardboard web. When the paper or cardboard web is more moist, its surfaces can also be smoothed better.
However, moistening has the disadvantage that it cannot be used together with the calibration calender that forms the calibration nip. When the paper or cardboard web is moistened again after calibration, i.e., after passing through the calibration nip, nonuniform thicknesses result across the width of the paper or cardboard web. This can possibly be attributed to the fact that the differences in thickness previously observed result again due to the swelling back of the areas condensed to differing extents.